Classification of asphaltenes via fractionation and the effect of heteroatom content on dissolution kinetics

The goal of this work is to provide characterization and a unified framework for understanding asphaltenes in crude oil. A fractionation technique that divides an asphaltene sample into different components based on polarity was developed. The morphology of the fractions varied from dense, shiny black particles that displayed a crystalline microstructure under SEM examination (the most polar fraction) to porous, dull-brown powders of a completely amorphous nature (the least polar fraction). Extensive studies using gel permeation chromatography, FTIR spectroscopy, and elemental analysis revealed no apparent structural differences between various fractions. However, substantially different dissolution characteristics were displayed by the fractions in a differential reactor with both toluene and an amphiphile/alkane micellar solvent. Fractions of higher polarity displayed lower dissolution rate constants and dissolved to a lesser extent than the lower polarity fractions. Analysis of asphaltene samples from eight different crude oils indicate that the fractionation technique may be a powerful tool for predicting the dissolution rate constant of an asphaltene sample and subsequently its difficulty of remediation. Further analysis of the fractions using ICP-90 and X-ray flourescence suggests that heteroatom content (especially metals such as iron, nickel, vanadium, aluminum, and nonmetals such as chlorine) plays a major role in determining the high polarity of asphaltenes. Treatment of the highest polarity fraction with the salt form of ethylenediamine tetraacetic acid (EDTA; a powerful metal-chelating agent) significantly increased dissolution rates when dodecyl benzene sulfonic acid (DBSA) was used in a heptane solution, suggesting that metal content may have a direct effect on dissolution rates. A slight increase in dissolution rate was observed with toluene as the solvent.